Re: directory archive format for pg_dump

On 16.12.2010 20:33, Joachim Wieland wrote:
> On Thu, Dec 16, 2010 at 12:48 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>> As soon as we have parallel pg_dump, the next big thing is going to be
>> parallel dump of the same table using multiple processes. Perhaps we should
>> prepare for that in the directory archive format, by allowing the data of a
>> single table to be split into multiple files. That way parallel pg_dump is
>> simple, you just split the table in chunks of roughly the same size, say
>> 10GB each, and launch a process for each chunk, writing to a separate file.
>
> How exactly would you "just split the table in chunks of roughly the
> same size" ?

Check pg_class.relpages, and divide that evenly across the processes.
That should be good enough.

> Which queries should pg_dump send to the backend? If it
> just sends a bunch of WHERE queries, the server would still scan the
> same data several times since each pg_dump client would result in a
> seqscan over the full table.

Hmm, I was thinking of "SELECT * FROM table WHERE ctid BETWEEN ? AND ?",
but we don't support TidScans for ranges. Perhaps we could add that.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com